The present invention is generally related to magnetically enhanced reactive ion etching equipment, and more particularly to the calibration of the magnetic field of such equipment.
Reactive ion etchers are used extensively by the microelectronics industry in the manufacturing of semiconductor and silicon based devices. Such devices include, for example, integrated circuits and micro-machined devices. Reactive ion etching is a dry etching process by which undesirable portions of a layer or film of one particular material are removed from a substrate wafer or layer of another material by chemical and/or physical interaction with a plasma etchant. For example, reactive ion etching may be used in conjunction with a mask layer to remove material in one or more layers beneath the mask layer in accordance with a pattern defined by the mask layer. In other words, certain etchants react with and remove the material in the layer or layers beneath the mask layer which are exposed to the etchant. The mask layer is substantially immune to the etchant""s effects and remains in place. Additionally, reactive ion etchers are used to remove mask layers while leaving substantially undisturbed the layers below the mask including those portions that may be exposed to the etchant by virtue of the pattern established in the mask layer. Of course, the selection of the etchant, among other factors, will determine the chemical and/or physical reactivity or neutrality of the reactant""s effect upon the various layers and mask utilized in any particular process.
Magnetically enhanced reactive ion etchers expose a wafer to a reactive plasma contained within a chamber which is additionally subjected to a controlled magnetic field conventionally provided by electromagnets. As used herein, magnetically enhanced reactive ion etchers include any of a variety of plasma based etchers wherein a controlled magnetic field, also referred to as a B-field, is impressed upon the plasma to control various plasma characteristics such as temperature, plasma uniformity and ion-bombardment energy. Process optimization therefore requires a repeatable, controllable B-field.
In FIG. 1 a typical reactive ion etching apparatus 100 is illustrated. Such an apparatus includes central plasma chamber 2 and a plurality of magnetic drive coils 10a, 10b, 12a and 12b symmetrically surrounding the chamber 2. Each coil is oriented orthogonally with respect to the two immediately adjacent coils such that the magnetic field passing through the center of each coil is substantially orthogonal to the magnetic field passing through the center of each immediately adjacent coil. Opposing coil pairs are established by the two sets of non-adjacent coils 10a, 10b and 12a, 12b. Wafers are passed into the chamber through the center of coil 12b and valve slit 6.
Turning to FIG. 2, an exemplary sectional view taken through opposing coil pair 11a, 11b is illustrated. The magnetically enhanced reactive ion etching apparatus 200 in this figure is illustrated without a chamber lid in place, which would be conventional when service maintenance such as wet cleaning, kit changes or magnetic calibration is being performed. Illustration of a conventional lid assembly 60 is shown in FIG. 3. In the present illustration, access to chamber liner 31 through the lid opening 32 at the top of the apparatus 200 is required for magnetic probe tool 50 conventionally utilized in taking magnetic field measurements during chamber calibration.
Chamber walls 30 which are manufactured from a non-magnetic material such as aluminum generally define the plasma chamber liner 31. Within the chamber is cathode 20, which during process operation is subjected to an RF signal by generator 41. Electrostatic chuck 22 is attached to cathode 20 and is employed for holding a semiconductor wafer in a reaction plasma chamber with a high level of accuracy during semiconductor processing.
Calibration tool 40 comprises a base portion 44 defining a locating feature 46 for accepting magnetic probe tool 50 including element 52. Element 52 may for example be a Hall device. Calibration tool 40 also includes standoff legs 42 which locate the tool to electrostatic chuck 22 at the desired height and orientation. With tool 40 properly located, a controlled location for probe tool 50 is established and the measurements of the magnetic fields generated by the coils will be repeatable.
Calibration of the apparatus 200 requires accurate measurements by the magnetic probe 50 of the magnetic field generated by each coil. Calibration of the coils may be required for example upon process changes requiring kit swapping or for such reasons as replacement of a coil driver or loss of data due to controller hard disk failures. In the former scenario, it is generally conventional practice to open the chamber and perform a variety of maintenance operations prior to releasing the apparatus for production use in the manufacturing environment. This includes venting of the chamber, removal of the chamber lid, gas distribution plate and all process kits. A wet cleaning of the chamber, lid and gas distribution plate as well as other ancillary maintenance operations are performed. This maintenance can take significant time and manpower resources. Eight to twelve hours of apparatus down time is common. In the latter scenarios, the same maintenance operations must be performed in conjunction with the conventional invasive calibration method. It is, however, generally desirable to avoid such otherwise unnecessary process steps.
As mentioned, calibration of the chamber magnetic field requires measurement of the magnetic field. This is accomplished by providing each coil in turn with a known DC drive voltage or current and taking a measurement of the magnetic field by the probe 50 via line 53. The known voltage or current can be applied to the coils manually by way of controlled voltage or current sources or automatically through coil drivers 70, 71 which control the voltage or current to coils 11a and 11b, respectively. Each coil driver 70, 71 respond to external input signals 72, 73, respectively, such as a commanded voltage or current level from a process controller (not shown). The process controller may take many forms, for example a dedicated microprocessor based process controller with operator interface allowing voltage and current level selections during a calibration routine, or a general purpose PC based process controller with conventional keyboard/mouse operator interfaces also providing for voltage and current level selections during a calibration routine. Data corresponding to the generated magnetic field vector is collected for each coil. If a manual process is followed, data may be read by an operator from a data acquisition display of a device interfaced with the probe 50. The manually read data is input to the process controller such as may be requested during a calibration routine executed by the controller. The process controller may also be adapted to automate the calibration process through data acquisition circuitry for monitoring and processing the signal from probe 50 element 52 during the calibration process via line 53.
It may be desirable to take readings of the magnetic field of each coil for both phases of coil excitation. That is to say one reading at a positive DC voltage and current and one reading at a negative DC voltage and current. An average of the absolute value of the two readings or an aggregate of the absolute value of the two readings may then be used in the further steps of the calibration process. Also, multiple readings for each coil taken at different voltage or current magnitudes may be taken depending upon the granularity of the calibration method employed by the process controller. In summary, magnetic field measurements would be taken in accordance with the methodology set forth by the process equipment manufacturer.
The magnetic field measurements provide information regarding the absolute and relative performance of each coil of the process apparatus. Measurements of any one coil that is substantially different from an expected reading may be evidence of a faulty coil or coil driver. Expected values for the magnetic field readings are generally provided by the manufacturer of the plasma etching apparatus and are accurate for a given orientation of the magnetic probe such as is established with the calibration tool previously described. Assuming the magnetic field measurements for the coils fall within acceptable ranges, the process controller utilizes the data to normalize the response of the system to a desired response. This may be accomplished for example by applying a function, weight, adjustment factor or other trimming function to a base input signal of the respective coil drivers. Generally it is desirable to normalize the magnetic field of each coil such that equivalent independent fields would be generated. The precise method of calibration is not critical to the invention but it suffices to say that the drive voltage or current of each coil drive would be commanded in accordance with the desired system response by virtue of the calibration.
Therefore, it is one object of the invention to provide a non-invasive method of calibrating the magnetic field of a plasma chamber.
It is a further object to provide a non-invasive method of determining the magnetic field within a plasma chamber.
In accordance with the present invention, a reactive ion etcher is provided having a plasma chamber surrounded by magnetic coils. Through a series of measurements of magnetic field strength at a first predetermined location substantially on the chamber lid and a corresponding series of measurements of magnetic field strength at a second predetermined location within the chamber, the external magnetic field is correlated to the internal magnetic field to establish a function which when applied to external magnetic field measurements taken at the first predetermined location yields the magnetic field inside the chamber at the second predetermined location. This indirect measurement of the magnetic field inside the chamber is then utilized in a calibration routine for establishing the response of the magnetic field drivers.